1. Field of the Invention
The present invention relates to an image-taking apparatus, and more particularly to an image-taking apparatus that takes in an image of a subject optically through a zoom lens system and then outputs it in the form of an electrical signal by means of an image sensor, and among others to an image-taking apparatus provided with a compact, lightweight zoom lens system.
2. Description of the Prior Art
Today, digital cameras are quite popular. Digital cameras dispense with silver-halide film, and use instead an image sensor such as a CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) sensor to convert an optical image into an electrical signal so that the optical image can be recorded and transferred in the form of digital data. A digital camera incorporates an image-taking apparatus provided with a lens system and an image sensor, and, in recent years, such image sensors have come to have increasingly large numbers of pixels. Correspondingly, the demand for high-performance image-taking apparatuses has been increasing greatly. Among others, compact image-taking apparatuses have been eagerly sought after that incorporate a zoom lens system that permits zooming without degrading image quality. In addition, in recent years, as the image processing performance of semiconductor devices and the like increases, more and more personal computers, mobile computers, cellular phones, personal digital assistants (PDAs), and the like have come to be internally or externally fitted with an image-taking apparatus having a zooming capability. This has been spurring on the demand for compact, high-performance image-taking apparatuses.
To make an image-taking apparatus compact, the zoom lens system incorporated therein needs to be made compact. Thus, many zoom lens systems have been proposed that aim at compactness. For example, so-called three-unit zoom lens systems composed of three lens units, namely, from the object side thereof, a first lens unit having a negative optical power, a second lens unit having a positive optical power, and a third lens unit having a positive optical power are proposed in the following patent publications:                Publication 1: U.S. Pat. No. 5,745,301        Publication 2: Japanese Patent Application Laid-Open No. 2001-343587This type of zoom lens system, composed of a small number of optical elements and having a comparatively simple movement mechanism, has the advantage of being suitable to be made compact and other advantages.        
Publications 1 and 2 disclose zoom lens systems in which aspherical surfaces are introduced in plastic lens elements. Using plastic lens elements makes introduction of aspherical surfaces easy, and also helps reduce costs. However, the zoom lens system disclosed in Publication 1 is composed of low-refractive-index aspherical-surface lens elements alone. Thus, disadvantageously, the aspherical surfaces have so great curvatures that they are difficult to produce. The zoom lens system disclosed in Publication 2 is composed of a large number of lens element. This makes it difficult to achieve compactness and cost reduction.